Rodrigo,

Having used the DS'420 quite a bit for new projects, reading your post made me realize that only once did i replace a std '51 by just plugging in a DS'420. The chip replaced was a i87C51. Worked well.

A few thoughts:

1) Ensure your board is not causing the DS'420 to be entering "bootloader" mode upon Reset;

2) If your target board has a serial port than you are just one 74HC125 away from being able to program the chip in circuit;

The User Guide is good reading and has information on both avoiding and entering bootloader mode.

Dallas/Maxim has a lot of appnotes regarding the use of the DS'420. Good Reading!

regards,
p